This invention relates generally to imaging of acoustic energy and in particular to laser-based ultrasound generation and detection equipment.
Ultrasound equipment is commonly used in medical imaging and for non-destructive evaluation of materials. Ultrasound analysis involves the propagation of energy through solids, liquids, and gases as acoustic waves; typically a pulse of acoustic energy is applied to an object to be imaged and reflected waves of the acoustic pulse are detected and processed for imaging and analysis. The spatial relationship and amplitude of the reflected waves provide information as to the location and nature of structures that reflected the acoustic energy in the object being analyzed.
Piezoelectric transducers are frequently used to generate ultrasound pulses transmitted into the object to be analyzed and to detect reflected waves received at the transducer. Piezoelectric devices require extensive electrical cabling which places practical limits on the number of pixels that can be placed in a transducer array, which in turn limits the resolution of the array.
Optical techniques have also been used for generation and detection of acoustic waves in ultrasound imaging. For example, energy from a laser beam focussed on the surface of an object to be examined can generate an acoustic pulse in the object. The return pulse of acoustic energy is typically detected optically through the use of interferometry. A review of such techniques is provided in the book Laser Ultrasonics--Techniques and Applications by C. B. Scruby and L. E. Drain (IOP Publishing Ltd 1990), which is incorporated herein by reference. Noninterferometric techniques of optical detection of ultrasound include the knife-edge and surface-grating techniques and techniques based on reflectivity and light filters. See "Optical Detection of Ultrasound" by J. P. Monchalin, IEEE Transaction on Ultrasonics, Ferroelectrics, and Frequency Control, UFFC-33, September 1986, which is incorporated herein by reference. These laser-based methods of ultrasound detection are much less sensitive, by several orders of magnitude, than conventional piezoelectric-based methods.
Another laser-based method for detecting sound waves has been suggested in the article "Laser Hydrophone" by Y. A. Bykovskii et al., in Sov. Phys. Acoust. 34, p 204, March 1988. In the Bykovskii et al. optical hydrophone, movement of the hydrophone membrane varies the power and/or the phase of a semiconductor laser in the hydrophone to generate changes in the amplitude of an optical signal. The Bykovskii sonar hydrophone is relatively inefficient and thus has low sensitivity.
It is accordingly an object of this invention to provide a laser transducer for detecting acoustic energy.
Another object of this invention is to provide a laser transducer for generating and detecting ultrasound pulses.
It is a further object of this invention to provide a laser ultrasound apparatus that is relatively compact and adapted to having a large number of transducer pixels in a compact transducer array.
A still further object of this invention is to provide a laser ultrasound apparatus that exhibits high sensitivity and a wide dynamic range.